A storage system is a computer that provides storage service relating to the organization of information on writable persistent storage devices, such as memories, tapes or disks. The storage system is commonly deployed within a storage area network (SAN) or a network attached storage (NAS) environment. When used within a NAS environment, the storage system may be embodied as a file server including an operating system that implements a file system to logically organize the information as a hierarchical structure of directories and files on, e.g. the disks. Each “on-disk” file may be implemented as a set of data structures, e.g., disk blocks, configured to store information, such as the actual data for the file. A directory, on the other hand, may be implemented as a specially formatted file in which information about other files and directories are stored.
The file server, or filer, may be further configured to operate according to a client/server model of information delivery to thereby allow many client systems (clients) to access shared resources, such as files, stored on the filer. Sharing of files is a hallmark of a NAS system, which is enabled because of its semantic level of access to files and file systems. Storage of information on a NAS system is typically deployed over a computer network comprising a geographically distributed collection of interconnected communication links, such as Ethernet, that allow clients to remotely access the information (files) on the filer. The clients typically communicate with the filer by exchanging discrete frames or packets of data according to pre-defined protocols, such as the Transmission Control Protocol/Internet Protocol (TCP/IP).
In the client/server model, the client may comprise an application executing on a computer that “connects” to the filer over a computer network, such as a point-to-point link, shared local area network, wide area network or virtual private network implemented over a public network, such as the Internet. NAS systems generally utilize file-based access protocols; therefore, each client may request the services of the filer by issuing file system protocol messages (in the form of packets) to the file system over the network identifying one or more files to be accessed without regard to specific locations, e.g., blocks, in which the data are stored on disk. By supporting a plurality of file system protocols, such as the conventional Common Internet File System (CIFS), the Network File System (NFS) and the Direct Access File System (DAFS) protocols, the utility of the filer may be enhanced for networking clients.
A SAN is a high-speed network that enables establishment of direct connections between a storage system and its storage devices. The SAN may thus be viewed as an extension to a storage bus and, as such, an operating system of the storage system enables access to stored information using block-based access protocols over the “extended bus”. In this context, the extended bus is typically embodied as Fibre Channel (FC) or Ethernet media adapted to operate with block access protocols, such as Small Computer Systems Interface (SCSI) protocol encapsulation over FC or TCP/IP/Ethernet.
A SAN arrangement or deployment allows decoupling of storage from the storage system, such as an application server, and some level of information storage sharing at the application server level. There are, however, environments wherein a SAN is dedicated to a single server. In some SAN deployments, the information is organized in the form of databases, while in others a file-based organization is employed. Where the information is organized as files, the client requesting the information maintains file mappings and manages file semantics, while its requests (and server responses) address the information in terms of block addressing on disk using, e.g., a logical unit number (lun).
In the example of the Write Anywhere File Layout (WAFL™) file system, by Network Appliance, Inc., of Sunnyvale, Calif., a file is represented as an inode data structure adapted for storage on disks. FIG. 1 is a schematic block diagram illustrating an exemplary on-disk inode 100, which preferably includes a meta data section 110 and a data section 150. The information stored in the meta data section 110 of each inode 100 describes a file and, as such, includes the type (e.g., regular or directory) 112 of the file, the size 114 of a file, time stamps (e.g., accessed and/or modification) 116 for the file and ownership, i.e., user identifier (UID 118) and group identifier (GID 120), of the file. The meta data section 110 further includes a xinode field 130 containing a pointer 140 that references another on-disk inode structure containing, e.g., access control list (ACL) information associated with the file or directory.
The inode 100 may also include a flags field 135 where various flags associated with the inode and the file system. The contents of the data section 150 of each inode may be interpreted differently depending upon the type of file (inode) defined within the type field 112. For example, the data section 150 of a directory inode contains meta data controlled by the file system, whereas the data section of a regular inode contains user-defined data. In this latter case the data section 150 includes a representation of the data associated with the file.
Specifically, the data section 150 of a regular on-disk inode may include user data or pointers, the latter referencing 4 kilobyte (KB) data block on disk used to store the user data. Each pointer is preferably a logical volume block number which is thereby facilitate efficiency among a file system and/or disk storage layer of an operating system when accessing the data on disks. Given the restricted size (e.g., 128 bytes) of the inode, user data having a size that is less than or equal to 64 bytes is represented in its entirety within the data section of an inode. However if the user data is greater than 64 bytes but less than or equal to 64 kilobytes (KB), then the data section of the inode comprises up to 16 pointers, each of which references a 4 KB block of data on disk. Moreover, if the size of the data is greater than 64 KB but less than or equal to 64 megabytes (MB), then each pointer in the data section 150 of the inode references an indirect inode that contains 1024 pointers, each of which references a 4 kilobyte data block on disk.
Some known file systems contain the capability to generate a snapshot of the file system. In the example of a WAFL-based file system, snapshots are described in TR3002 File System Design for a NFS File Server Appliance by David Hitz, et al., published by Network Appliance, Inc. and in U.S. Pat. No. 5,819,292 entitled METHOD FOR MAINTAINING CONSISTENT STATES OF A FILE SYSTEM AND FOR CREATING USER-ACCESSIBLE READ-ONLY COPIES OF A FILE SYSTEM, by David Hitz, et al., which are hereby incorporated by reference.
“Snapshot” is a trademark of Network Appliance, Inc. It is used for purposes of this patent to designate a persistent consistency point (CP) image. A persistent consistency point image (PCPI) is a point-in-time representation of the storage system, and more particularly, of the active file system, stored on a storage device (e.g., on disk) or in other persistent memory and having a name or other identifier that distinguishes it from other PCPIs taken at other points in time. A PCPI can also include other information (metadata) about the active file system at the particular point in time for which the image is taken. The terms “PCPI” and “snapshot” shall be used interchangeably through out this patent without derogation of Network Appliance's trademark rights.
When the file system generates a snapshot of a given file system, a snapshot inode is generated as shown in FIG. 3. The snapshot inode 305 is, in essence, a duplicate copy of the inode for the inode file 205 of the file system 200. Thus, the exemplary file system structure 200 includes the inode file indirect blocks 210, inodes 217, indirect blocks 219 and file data blocks 220A–C as in FIG. 2. When a user modifies a file data block, the file system layer writes the new data block to disk and changes the active file system to point to the newly created block.
FIG. 4 shows an exemplary inode file system structure 400 after a file data block has been modified. In this illustrative example, file data block 220C was modified to file data block 220C′. When file data block 220C is modified to file data block 220C′, the contents of the modified file data block are written to a new location on disk as a function of the exemplary WAFL file system. Because of this new location, the indirect block 419 must be rewritten. Due to this changed indirect block 419, the inode 417 must be rewritten. Similarly, the inode file indirect block 410 and the inode for the inode file 405 must be rewritten. Thus, after a file data block has been modified the snapshot inode 305 contains a point to the original inode file indirect block 210 which in turn contains pointers through the inode 217 and an indirect block 219 to the original file data blocks 220A, 220B and 220C. However, the newly written indirect block 419 includes pointers to unmodified file data blocks 220A and 220B. The indirect block 419 also contains a pointer to the modified file data block 220C′ representing the new arrangement of the active file system. A new inode for the inode file 405 is established representing the new structure 400. Note that metadata (not shown) stored in any snapshotted blocks (e.g., 305, 210, and 220C) protects these blocks from being recycled or overwritten until they are released from all snapshots. Thus, while the active file system inode for the inode file 405 points to new blocks 220A, 220B and 220C′, the old blocks 210, 217, 219 and 220C are retained until the snapshot is fully released.
After a snapshot has been created and file data blocks modified, the file system layer can reconstruct or “restore” the file system inode structure as it existed at the time of the snapshot by accessing the snapshot inode. By following the pointers contained in the snapshot inode 305 through the inode file indirect block 210, inode 217 and indirect block 219 to the unmodified file data blocks 220A–C, the file system layer can reconstruct the file system as it existed at the time of creation of the snapshot.
Storage operating systems implementing snapshots typically provide a command to enable a user or administrator to obtain information about snapshot consumption of available disk space. This command, which may be executed via a command line interface (CLI) or a graphical user interface (GUI), typically displays a percentage of the disk space utilized by the snapshots associated with the storage system. However, a noted disadvantage of the use of such a snapshot inquiry command is that a significant amount of time and processing power must be expended to compute the file system usage of each of the snapshots at the execution of the snapshot inquiry command. In some systems having many snapshots this time may exceed a half an hour.
Another noted disadvantage is that storage operating systems implementing snap-shots typically may not contain a mechanism to determine how many blocks have changed between any two snapshots. This information may be particularly useful in connection with certain mirroring (also termed “replication”) techniques that utilize snap-shots as point in time images for reconciliation purposes. One example of such mirroring or replication techniques is described in U.S. patent application Ser. No. 10/100,967, entitled SYSTEM AND METHOD FOR DETERMINING CHANGES IN TWO SNAPSHOTS AND FOR TRANSMITTING CHANGES TO A DESTINATION SNAPSHOT, by Michael L. Federswitch, et al., the contents of which are hereby incorporated by reference.